MECHANICAL AND MORPHOLOGIC INVESTIGATION OF THE TENSILE-STRENGTH OF ABONE-HYDROXYAPATITE INTERFACE

For load-bearing calcium-phosphate biomaterials, it is important to un derstand the relative contributions of direct physical-chemical bondin g vs. mechanical interlocking to interfacial strength. In the limit of a perfectly smooth hydroxyapatite (HA) surface, a tensile test of the bone-HA interface affords an opportunity to isolate the bonding contr ibution related to HA surface chemistry alone. This study measured the bone-HA interfacial tensile strength for highly polished (similar to 0.05 mu m alumina) dense HA disks (5.25 mm in diameter, 1.3 in mm thic kness) in rabbit tibiae. Each of five rabbits received four HA disks, two per proximal tibia. Pull-off loads ranged from 3.14 +/- 2.38N at 5 5 days after implantation to 18.35 +/- 11.9N at 88 days; nominal inter facial tensile strengths were 0.15 +/- 0.11 MPa and 0.85 +/- 0.55 MPa, respectively. SEM of failed interfaces revealed failures between HA a nd bone, within the HA itself and within adjacent bone. Tissue remnant s on HA were identified as mineralized bone with either a lamellar or trabecular structure. Oriented collagen fibers in the bone intricately interdigitated with the HA surface, which frequently showed breakdown at material grain boundaries and a rougher surface than originally im planted. Mechanical interlocking could not be eliminated as a mode of tissue attachment and contribution to bone-HA bonding, even after impl anting an extremely smooth HA surface. (C) 1997 John Wiley & Sons, Inc .